Towards the end of the ninth century, Pope Stephen VI had the body of his predecessor Formosus exhumed, dressed in its sacerdotal attire, and propped up in the council chamber to face trial for trumped-up charges of perjury and usurping the bishopric of Rome. During the "prosecution", Stephen hurled abuse at the corpse, and tauntingly challenged it to reply to his accusations. Perhaps taking silence as an admission of guilt, Stephen duly convicted Formosus, and had his body mutilated before throwing it into the Tiber. This particular pope was, evidently, something of a diva.

A similar thing happened last week with the publication of Richard Seymour's book Unhitched: The Trial of Christopher Hitchens. As the title implies, it's an all-out assault on the late polemicist in which he is attacked for, amongst other crimes, his "career-minded avarice", his "misogyny", "his many plagiarisms", his "ferocious American nationalism", his "profoundly colonial" outlook, his "closeted sympathy for Thatcherism", his "theophobia", his "bigoted attitude towards Muslims", his "downright racism", and for acting as "an amanuensis of the Bush administration".

Given that the "trial" is posthumous, Seymour comes across every bit as churlish and opportunistic as Pope Stephen. But readers should consider two points before judging Seymour too harshly. First of all, the book was commissioned six months before Hitchens's death, so all Fred Inglis's talk of Seymour dancing on the "fresh grave" seems a little unfair. Secondly, this kind of blood-and-guts diatribe is precisely the sort of thing that got Hitchens excited. He wrote similarly damning screeds against Mother Teresa, Bill Clinton and Henry Kissinger, and certainly didn't observe a tactful period of mourning before remarking of the late Jerry Falwell that "it's a pity there isn't a Hell for him to go to". Hitchens may not have liked Seymour's book, but he would have relished the chance to lock horns in the aftermath.

And he would have had much to criticise. Seymour is a purist when it comes to Marxism; he will brook only one interpretation (his own), pronouncing Hitchens as guilty of "philistinism" and "vulgarisation", as though Marxism were free from divergent schools of thought. More seriously, he demonstrates an uncritical overreliance on biased sources; most notably the testimony of Tariq Ali, whose personal animosity since his falling out with Hitchens is well known. In short, Seymour's book is unashamedly partisan, meaning that his analytical objectivity is compromised. This isn't so much a trial as a show trial.

That said, it's much better than his critics have suggested. Seymour has a methodical, assured style, and he understands as well as Hitchens that the occasional rambunctious self-indulgence makes for an enjoyable read. There can also be no denying that Seymour knows his subject. His familiarity with Hitchens's work is of a kind only ever attained by a true aficionado or determined adversary.

Central to Seymour's thesis is the notion of an ideological consistency throughout Hitchens's life; that he always was, essentially, a "man of the right" (an accusation which, if true, would surely undermine Seymour's charges of apostasy). But Seymour is extremely effective when it comes to pointing out Hitchens's many discrepancies in his views on American foreign policy and military intervention, particularly with regard to the Bosnian war following the breakup of Yugoslavia in 1992. He adroitly identifies Hitchens's vacillations over the NATO air strikes in Kosovo, and his use of Hitchens's own advice from Letters to a Young Contrarian to attack his own later tendency to speak of the US military in the first person plural is a shrewd move.

But looked at from another perspective, there is perhaps more consistency in Hitchens's position than Seymour admits. Hitchens, for instance, always mistrusted pacifism. In 1965 Hitchens was calling for the withdrawal of British troops from Aden and South Yemen on anti-imperialist grounds. At the same time, he supported intervention in Zimbabwe against white supremacists in the Rhodesian Front, albeit feeling "uneasy" whilst doing so. He blamed American foreign policy for the partition of Cyprus in his book Hostage to History, but was scornful of British "inaction" during the Turkish invasion of 1974. He even went so far as to rejoice in the ETA's assassination of Luis Carrero Blanco in 1973.

So one might say that Hitchens was consistent in his inconsistency - a point he freely acknowledged - for the simple reason that no two conflict situations are precisely alike. Hitchens's rallying cry "out of Aden, into Rhodesia" was not, therefore, necessarily hypocritical.

Much of this comes down to individual moral values. Hitchens would probably agree with Seymour's conclusion that he began to "identify the United States Armed Forces as a human rights detachment, an antigenocide task force, and a vector for democracy", but whether or not this is an indictment is disputable. It is certainly damaging if you accept Seymour's interpretation that he was motivated by a latent imperialism, but this is unconvincing. You don't have to be pro-war to recognise in Hitchens's argument an ideological coherence firmly grounded in a hatred of totalitarianism. When Hitchens stated that this was his sole consistency, he had a point.

But Seymour won't have it. For instance, he insists that Hitchens's support for the Falklands War was a manifestation of his "instinctive Thatcherism" and "faith in empire". But isn't this rather unlikely given Hitchens's view of extreme nationalism as "toxic", as expressed unambiguously in his memoir Hitch-22? Rather, Hitchens's support of the Falklands conflict was based on an overt animosity towards the fascistic nature of the Argentine junta, a perfectly satisfactory explanation that Seymour unfairly dismisses as mere "left gloss".

Seymour makes much of the nationalistic fervour that galvanised public enthusiasm for Thatcher in the wake of the crisis. But that has nothing whatsoever to do with Hitchens's stance in 1982, which seems to me entirely consistent with his later support of the war in Iraq when understood from a position of liberal internationalism. Whether Seymour chooses to accept it or not, Hitchens's shift to the right was motivated by a desire to defend the principles of the left.

This is why a reductive left/right dichotomy is unhelpful when it comes to engaging with Hitchens's later career. His "defection" is appealing as a convenient narrative, but it doesn't do justice to the substance of his work. Seymour accuses Hitchens of propagating a new "Jeffersonian" imperialism, or a "neoliberal imperialism with a faint leftist patina" but the essay he cites in support is not really a paean to empire at all. Again and again, he tries to shoehorn Hitchens into a fixed, familiar category, but is only able to do so through careful omission. He speaks of Hitchens's "savage rhetoric of conquest" but supplies no examples. He tells us that Hitchens's attack on Edward Said in 2003 was "a signal that he would never again write the things he once had about Palestine. And indeed he never again did". But here's Hitchens writing for Slate in 2009:

Surely nobody will be so callous as to say that there is less despair among Palestinians today - especially since the terrible events in the Gaza Strip and the return to power of the Israeli right wing as well as the expansion of Jewish-zealot settler activity.

Seymour is right that the emphasis of Hitchens's targets altered in the last decade of his life, but it is simply not true to suggest that he lost all sympathy for the Palestinian cause. It is revealing that Seymour dismisses Hitchens's 2006 piece "Overstating Jewish Power" as "equivocal", even though in that very article Hitchens reasserts his belief that "the Israeli occupation has been a moral and political catastrophe", and goes on to cite a litany of examples of how the Israeli government has been complicit in various human rights abuses through its morally dubious foreign policy.

Perhaps what Seymour means by "equivocal" is "insufficiently aligned with my own particular views". And this strikes me as the major failing of Unhitched. This would have been a more persuasive book had Seymour restricted himself to his perfectly legitimate criticisms of Hitchens's arguments. He is able to refute successfully some of the more egregious misconceptions, such as the belief that Abu Musab al-Zarqawi was an al-Qaeda agent working in tandem with Saddam Hussein before the Iraq war. Likewise, Hitchens's description of Northern Irish paramilitaries as "religious" in his book god is Not Great is deservedly ridiculed. But these valid critiques are marred by Seymour's repeated insistence that Hitchens was a shoddy writer, a view that simply won't stand up to serious scrutiny.

Critics of Hitchens have always been notoriously reluctant to concede the obvious truth, that whatever one might think of his ideas he was one of the finest rhetoricians of our time. There are nods to Hitchens's polemical expertise in Unhitched, but they are sporadic and grudging. Seymour acknowledges his "eloquence" and "rhetorical power" but, tellingly, only when Hitchens's views mirror his own. He avers that Hitchens's intellect was "greatly overvalued in his later years"; coincidentally, it seems, when he began writing articles that differed from Seymour's worldview. Draw your own conclusions.

One of Seymour's tactics is the wholly unsubstantiated imputation of plagiarism. Offering by way of example Hitchens's review of Edward Said's Orientalism, Seymour asserts that "much of the article is actually plagiarised from the book it is allegedly reviewing". Rather than provide any concrete examples of his own, he instead footnotes an article by Clare Brandabur in which this claim is originally made. Here is Brandabur's argument:

Throughout his essay, he takes lines from Orientalism without letting the reader know they are not his own. For example, when he says that Lord Macaulay was "a near perfect illustration of the sentence (which occurs in Disraeli's novel Tancred) 'The East is a career'." That line, correctly attributed, occurs on page 5 of Orientalism. But you would not know that from Hitchens' text. From there he moves to discuss Karl Marx, again taking passages in which Said quotes Marx quoting a stanza from Goethe (pages 153-4 of Orientalism).

Brandabur simply does not know what she is talking about. Hitchens is perfectly at liberty to quote sources that Said has also used. Brandabur's examples only serve to demonstrate the exact reverse of what she is trying to prove, that Hitchens has not poached anybody else's work and passed it off as his own.

In his dogged quest to discredit Hitchens's professional integrity, Seymour also cites John Barrell's review of Hitchens's book Thomas Paine's Rights of Man. Barrell, it is claimed, had "detected plagiarism" in the text. Interesting, then, that in the review in question Barrell (no fan of Hitchens) makes the admission that "there is of course no question of plagiarism". Seymour omits this aspect of Barrell's review because it doesn't serve his agenda. It reminds me of the rhetorical question Seymour posed in his 2005 essay "The Genocidal Imagination of Christopher Hitchens": "why highlight a report to support your point when it doesn't say what you claim it does?"

Barrell indeed argues that Hitchens "depends heavily" on John Keane's 1995 biography of Paine, and cites passages in which Hitchens has clearly garnered some of the historical details that underpin his writing. He makes the valid observation that "Hitchens's debt to Keane is palpable", but Hitchens acknowledges as much himself. If plagiarism was his intention, one might suppose he would have kept quiet about his source.

A low point is when Seymour stoops to the unfounded accusation of racism. This is especially ignoble given Hitchens's lifelong passion for opposing racism in all its forms. In his memoir Hitchens writes: "I would not have as a friend somebody whom I suspected of that prejudice", and to doubt him seems plain boorish.

Later, Seymour connects "racism" with what he calls "theophobia" (an unfortunate choice of word given that a fear of God is considered to be a virtue in Christian doctrine). He argues that in Hitchens's case "antitheism as an element in his politics and idiolect really began in and around 2005". Anyone familiar with Hitchens's body of work will appreciate the extent of this inaccuracy. Seymour maintains that in the wake of the fatwa on Salman Rushdie "Muslims living in the imperialist societies found themselves increasingly pigeonholed into identarian boxes... as their religious identity was itself racialised", an over-simplification which enables him to conflate race and religious ideology. This is spurious. It accounts for the common misuse of the neologism "Islamophobic" to disparage those who have legitimate reservations about theocracy and religion generally. Seymour (re)defines "Islamophobia" to suit his purposes, with a little help from the Runnymede Trust, but his application of the term to Hitchens is a form of posthumous libel.

Unhitched is unlikely to be the last book-length attack on Hitchens, but in spite of such efforts Hitchens's legacy is assured. A few hungry bark beetles cannot topple a redwood. Seymour's belief that Hitchens's "literary flair... declined in proportion to his political nous" is wishful thinking; a well-argued untruth. As a strategy, it cannot help but misfire. As one of the most prolific and brilliant political writers of our time Hitchens's gifts were prodigious, and if there is a guilty verdict to be reached in this "trial", it must surely be to the charge of "not being in complete agreement with Richard Seymour". I should think Hitchens would be happy enough with that.